The hazard ratio for LR with triple-negative phenotype was 15.4 (95% CI, 2.5-91).

All 3 of the triple negative patients who developed LR had previously received systemic chemotherapy.

On multivariate analysis, triple-negative phenotype was the only predictor of LR, with borderline statistical significance (p = 0.052, log-rank test) after adjusting for tumor grade.

Effect of tumor grade on risk of LR was not significant (p = 0.33, log-rank test) after adjusting for triple-negative status.

All LR patients were successfully salvaged with mastectomy.

Author's Conclusions

In this study, patients with triple-negative breast cancer had a significantly higher LR rate than patients with other receptor subtypes following 3D-APBI to 32 Gy.

Caution should be used in deciding whether or not to treat triple negative breast cancer patients with 3D-APBI outside of the setting of a clinical protocol.

Additional prospective studies are needed to assess the long-term efficacy of this approach, and whether patients with triple-negative disease might be better treated with whole-breast irradiation.

Clinical Implications

This study was a subset analysis of a prospective clinical trial of dose escalation in 3D-APBI as part of breast conservation therapy for early stage breast cancer.

The authors found that triple negative breast cancer patients treated with APBI had a significantly elevated 5 yr LR rate of 32.5% compared to 3% in non-triple negative patients.

This rate of LR seen with APBI is inferior to the results observed by Nguyen et al. in triple negative patients treated with WBI (7% at 5 years). The observation in the current study that the majority of failures were not within or near the tumor bed supports the utility of WBI in this population.

The published ASTRO consensus guidelines indicate that only women with ER + tumors are deemed “suitable” for APBI outside of the context of a clinical trial.

This study provides prospective clinical data supporting the recommendation that triple negative breast cancer patients are unsuitable for APBI unless as part of a clinical trial.

Limitations of the study include:

The study had a limited sample size of only 99 patients, 10 of which were triple negative.

There were also a small number of events with only 5 patients developing LR with the current follow-up.

APBI is currently being evaluated in the ongoing NSABP B-39/RTOG 0413 trial which is a phase 3 randomized trial comparing APBI to WBI for women with early stage breast cancer undergoing breast conservation therapy.

The results of that study should better inform the selection of appropriate patients for APBI.